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    MAMMALS MAMMALS? 2008 Editorial Sol 90

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    Britannica Illustrated Science Library: Mammals 2008

    Printed in China

    www.britannica.comMammalsContents

    Origin and

    Evolution

    Page 6

    What They

    Are Like

    Page 18

    Behavior and

    Life Cycle

    Page 32

    Relationship

    with People

    Page 80

    Page 60

    Diversityhunters and gatherers to a society based on

    agriculture. At that time, humans began to

    benefit from the meat and milk products of

    small mammals and to use large animals for

    labor. The first animals to be domesticated

    were sheep (about 9000 BC) in the Middle

    East. Pigs, cows, goats, and dogs followed.

    However, the great majority of mammal

    species continue, even today, to live in the

    wild.

    T

    here are 5,416 known mammal species

    distributed over different land and

    aquatic environments. Despite the

    characteristics that make them part of the

    same class, their diversity is such that the

    smallest of them, the shrew, may weigh only

    one tenth of an ounce (3 g), and the largest,the blue whale, can reach 160 tons. But their

    diversity is also evident in their adaptation to

    different environments. There are mammals

    that run and others that glide—some fly, and

    others jump, swim, or crawl. Most aquatic

    mammals have suppressed the development

    of hair or fur, replacing it with thick layers of

    fat. The rigors of low temperatures have

    made some animals—such as polar bears,dormice, and certain bats—exceptions to the

    vital law of homeothermy, as they spend the

    winter sunk in deep sleep to save energy.

    WALES

    Land of green meadows

    and gentle hills, Wales

    is famous the world

    over for the quality of

    its wool production.

    Seals, dolphins, bats, and chimpanzees all

    have upper limbs with similar bones, but the

    environmental niche they occupy has made

    seals develop flippers, dolphins fins, bats

    wings, and chimpanzees arms. Thus from the

    polar tundra to the dense tropical jungle,through the deep oceans and high mountain

    lakes, the whole Earth has been populated

    by thousands of mammal species.

    But this marvelous animal world has

    been disturbed by its most numerous

    species—humankind. Indiscriminate

    hunting, illegal trade, deforestation,urbanization, massive tourism, and pollution

    have left more than a thousand species

    (many of them mammals) endangered or

    vulnerable. However, science allows us to

    understand nature's many wonders, and it

    can help us respect the world's ecological

    balance. In this book, which includes

    dazzling photographs and illustrations, we

    invite you to discover many details of

    mammals' lives: their life cycles, their social

    lives, their special features, and their

    characteristics, from those of the greatest

    friend of them all, the dog, to the mysterious

    and solitary platypus.

    Mammals began to dominate the

    Earth about 65 million years ago.

    Without a doubt, modern humans

    are the most successful mammals—they

    occupy all the Earth's habitats! Their

    domestic coexistence with other species

    began barely 10,000 years BC, when human

    culture transitioned from a world of nomadic

    Unique and

    DifferentOrigin and Evolution

    P

    olar bears are all-around

    athletes, as agile in the water as

    they are on land. Excellent

    swimmers, they move at a speed

    of 6 miles per hour (10 kmh)

    using a very rapid stroke. They can rest

    and even sleep in the water. Like all

    mammals, they have the ability to

    maintain a constant temperature. This

    allows them to tolerate the extreme cold

    of the Arctic ice. Here we will tell you

    many more things about the particular

    properties that distinguish mammals from

    the rest of the animals. Did you know that

    mammals appeared on Earth at almost

    the same time as dinosaurs? Since they

    were unable to compete with the large

    reptiles of the time, at first they were very

    small, similar to mice. Turn the page and

    you will discover many more things.

    MILLIONS OF YEARS AGO . . 8-9

    NAMES AND GROUPS 10-13

    WHAT IS A MAMMAL? 14-15

    CONSTANT HEAT 16-17

    POLAR BEARS

    Also called the white bear,they are without a doubt

    “Lords of the Arctic.”

    Nevertheless, they are on

    the road to extinction.Morganucodon

    T

    he origin of mammals lies in the Triassic Period a little more than 220 million years

    ago when, in the course of terrestrial evolution, new groups of animals appeared.

    Their history can be reconstructed in broad outline through the study of fossils.

    Among them is the morganucodon, an animal of which we have found numerous remains.

    Millions of Years Ago . . .

    Clade

    Group

    Subgroup

    Family

    Genus

    Weight

    1 to 1.8 ounces

    (30-50 g)

    6 inches (15 cm)

    Millions

    of Years

    KEY

    EAR

    Mandible Squamosal Angular Subangular Malleus Incus Stapes

    (Hammer) (Anvil) (Stirrup)

    Mammaliaformes

    Monotremes

    Multituberculates Marsupials Placental Mammals

    Primitive

    Therians

    0

    100

    200

    Period

    EXTINCT

    FAMILIES

    TRIASSIC JURASSIC CRETACEOUS TERTIARY

    Mammaliaformes

    Synapsids

    Triconodonts

    Cynodont

    Morganucodon

    PRIMITIVE

    REPTILES

    Resembled mammals

    in the bones of their

    back, neck, and hips,which allowed them

    to stand more

    upright. They

    replaced their teeth

    only once and had a

    much larger brain

    than today's reptiles.

    MAMMALS

    The cranium is larger, the

    mandible is formed by a

    single bone, the ear is

    articulated, and the teeth are

    of different shapes and sizes.

    HUMERUS

    is bigger, allowing

    greater mobility

    of the forelimbs.

    INTERIOR FOSSA

    The transformation of the

    mandibular bones into

    those of the modern mammal

    is not yet complete.

    COAT

    Although mammals are

    warm-blooded and can

    keep their body

    temperature constant,their fur coats protect

    them from the cold.

    EPICONDYLE

    articulates with the

    humerus and connects

    to the forelimbs.

    PATELLA

    is the knee,which connects

    the femur with

    the tibia and the

    fibula.

    TROCHANTER

    is the part of the

    femur where

    muscles that

    assist locomotion

    are inserted.

    ACETABULUM

    connects to the

    lumbar vertebrae

    and pelvis.

    TAIL

    is shorter than

    that of today's

    rodents and

    pointed.

    LUMBAR

    VERTEBRAE

    do not have ribs

    and withstand the

    body's twisting.

    POSTURE

    The bones of the back,neck, and hip allowed it

    to stand more upright.

    SCAPULA

    connects the legs with

    the lumbar vertebrae.

    MONOTREMES

    STEROPODON GALMANI

    Reptile Mammal

    MARSUPIALS

    DIPROTODON AUSTRALIS

    PLACENTAL MAMMALS

    ZALAMBDALESTES

    MAMMALIAFORMES

    Had differentiated

    dentition, with incisor,canine, and molar

    teeth. They also

    developed an extensive

    secondary palate, an`d

    the mandible was

    formed by the dentary

    bone. The posterior

    bones, which

    articulated with the

    cranium, had become

    smaller.

    8 ORIGIN AND EVOLUTION

    FROM REPTILE TO MAMMAL

    Incus

    (Anvil)

    Stapes

    (Stirrup) Inner Ear

    Malleus

    (Hammer)

    Incisors

    Like mammals, they

    had a single dentary

    bone (mandible).

    Single

    Dentary Bone

    (Mandible)

    Mandible

    formed by

    various bones

    EAR

    Large and articular,it approximates

    those of mammals.

    EAR

    Inner ear

    Three tiny bones

    Stapes (Stirrup)

    Incus (Anvil)

    Malleus (Hammer)

    HANDS

    8 carpal

    bones

    5 metacarpals

    5 proximal

    phalanges

    5 medial

    phalanges

    4 distal

    phalanges

    FEET

    7 tarsal

    bones

    5 metatarsals

    5 phalanges

    5 medial

    phalanges

    4 distal

    phalanges

    MOLAR TEETH

    Triangular in

    shape, the prior

    formation of

    incisors is

    reversed, and they

    increase to four.

    Canines

    Premolars Molars

    Multituberculates

    These Mesozoic mammals had

    features similar to those of living

    rodents. They had incisors in the mandible

    as well as in the cranium that grew

    continuously. There were both arboreal

    and digging multituberculates, and their

    fossil remains have been found on every

    continent except Australia and Antarctica.T

    he mammals class is divided into two subclasses:

    Prototheria, which lay eggs (like other classes such

    as birds), and Theria. The Theria, in turn, are

    divided into two infraclasses—Metatheria (marsupials),which grow to viability within a marsupium, or pouch,and Eutheria (placental mammals), whose offspring

    are born completely developed and who today

    represent the great majority of living mammal

    species, including humans.

    Prototheria

    Order Monotremata

    Oviparous mammals (Monotremata) are the

    oldest of all known groups. It is believed that their

    origin could be independent from that of other

    mammals and that they descend directly from the

    Synapsid reptiles of the Triassic Period (more than

    200 million years ago).

    Monotremes are the only mammals that lay eggs.

    However, the shape of their craniums, the

    presence of hair, and, of course, mammary glands

    show that they belong to the mammal group. The

    mammary glands lack nipples, so the young have

    to lick milk from a tuft of hair.

    The only living representatives of this order are

    echidnas and the platypus. The platypus is a

    unique species that, because of its similarity to

    birds, was impossible to classify zoologically

    for a long time.

    Theria

    Infraclass Metatheria

    The principal characteristic of metatherias, or

    marsupials, is the way they reproduce and develop. They

    have a very short gestation period compared to other

    mammals (the longest is that of the giant gray kangaroo,only 38 days), which means that their newborn are not

    very developed but have bare skin and eyes and ears

    that are still in the formative stage—although they have

    a sense of smell, a mouth, and digestive and respiratory

    systems adequate for survival. When they are born, they

    crawl across their mother's abdomen in search of her

    mammary glands. Kangaroo offspring climb to the edge

    of the mother's pouch (marsupium). They then crawl in

    and affix themselves to one of the mammary glands,from which they feed until they complete development

    and leave the pouch.

    ECHIDNA

    Family Tachyglossidae

    Also known as the “spiny

    anteater” because it feeds

    on ants and termites that it

    catches with its tongue. Its

    skin has hair and spines.

    AUSTRALIA

    TASMANIAN DEVIL

    Family Dasyuridae

    The largest of the carnivorous

    marsupials became extinct in

    Australia 600 years ago, but it

    survives on the island of Tasmania. It

    is a predator the size of a small dog.

    ALMOST PATRIMONY

    Unlike the rest of the world,almost no placental mammals live

    in Australia and its neighboring

    islands. The island continent

    possesses 83 percent of the unique

    (endemic) species of mammals.

    OPOSSUMS

    Family Didelphidae

    They spend most of

    their lives perched in

    trees and are very timid.

    MAMMALS 11

    4 SPECIES KNOWN

    CURRENTLY

    300 SPECIES EXIST.

    OVER

    AUSTRALIA SOUTH

    AMERICA

    Mammals Colonizing the World

    HORNY BEAK

    is used to

    rummage in

    riverbeds and mud

    in search of food.

    FINS

    Platypuses use

    their limbs to

    swim.

    PLATYPUS

    Family Ornithorhynchidae

    A monotreme with semiaquatic

    habits. Its feet and tail possess

    membranes that make it palmate,which is useful for swimming. It feeds

    off any living thing it finds at the

    bottom of Australia's rivers or lakes

    by rummaging with its horny beak.

    GEOGRAPHICALLY CONFINED

    Platypuses and echidnas are found only

    in Oceania—the platypus only on

    Australia and the echidna (of which

    there are four species) also on the

    islands of Tasmania and New Guinea.

    The first fossils of marsupials and

    placental mammals were found in

    rocks dating from the late Jurassic and

    the earliest part of the Cretaceous

    periods. At that time, America, Africa, and

    Australia were united in a single continent

    (Gondwana) and were beginning to

    separate. But the placental mammals

    evolved further, and at the beginning of

    the Eocene Period (56 million years ago),opossums were the only representatives

    in America of marsupials, which otherwise

    prospered only in Australia's particular

    climate and geographic isolation.

    Names and Groups

    10 ORIGIN AND EVOLUTION

    Order Dasyuromorphia

    Order Didelphimorphia

    Order Diprotodontia

    Order Microbiotheria

    Order Notoryctemorphia

    Infraclass Metatheria

    Order Monotremata

    Order Paucituberculata

    Order Peramelemorphia

    Subclass

    PrototheriaMAMMALS 13

    GIRAFFE

    Order Artyodactilae

    These are the tallest of living land

    animals—they can be over 18 feet

    tall (5.5 m). They are herbivores.

    Their blood pressure is almost twice

    that of other large mammals, and

    their tongues are over 18 inches

    (0.5 m) long. They live in Africa.

    MANDRILL

    Order Primates

    Weighing up to 120 pounds (55 kg),these are the largest monkeys in the

    world. The males are much larger

    than the females, and they have a

    brilliantly colored face, with deep

    grooves running down both sides

    of their snout. Mandrills live in

    Africa's tropical zones. They

    are omnivores, eating

    anything from grasses to

    small mammals.

    SEALS

    Order Carnivora

    Along with elephant seals, they make

    up the Pinnipedia suborder. They

    move very clumsily on land, but they

    are very good swimmers. They

    feed on fish and crustaceans

    and prefer to inhabit

    marine waters near

    the poles, although

    they reproduce on

    dry land.

    Infraclass Eutheria

    Commonly called placental mammals, they are the typical

    mammals. They probably began diversifying during the Cretaceous

    Period (65-150 million years ago) from a different line of the

    metatherians. They are characterized by the fact that their

    embryos are implanted in the uterine cavity and develop an outer

    layer of cells in close union with the maternal body, the placenta.

    They receive nutrients directly from the placenta during their

    development until they are born with their vital organs (except for

    those responsible for reproduction) fully formed.

    RACCOON

    Order Carnivora

    Live in forests near rivers.

    These carnivorous hunters

    and climbers live in North

    America.

    SKIN

    A fur coat and

    subcutaneous fat

    protect the animal

    from extreme cold.

    NECK

    allows them to

    reach the

    highest leaves.

    Jurassic Beaver

    Scientists thought that mammals were able to

    conquer the Earth only after dinosaurs became

    extinct. But the recent find of a fossil of this

    beaver in China suggested that, by the Jurassic

    Period, when the giant reptiles were at their

    peak, mammals had already diversified and

    adapted to water ecosystems 100 million years

    earlier than had been believed. The

    Castorocauda lufrasimilis lived 140

    million years ago.

    ANTARTICA AFRICA

    EUROPE AMERICA ASIA

    OCEANIA

    THROUGHOUT THE WORLD

    The eutherians, or placental mammals, are

    the most important group of mammals

    because of the number of living species they

    represent. Their geographic distribution

    covers almost the entire planet, including on

    and beneath bodies of water and polar areas.

    These animals cover a wide range of

    ecosystems and forms of life and make up 19

    orders of viviparous placental mammals.

    SPECIES OF

    EUTHERIANS.

    4,000

    THERE ARE OVER

    Order Artiodactyla

    Order Carnivora

    Order Cetacea

    Order Chiroptera

    Order Dermoptera

    Order Hyracoidea

    Order Insectivora

    Order Lagomorpha

    Order Macroscelidea

    Order Perissodactyla

    Order Pholidota

    Order Primates

    Order Proboscidea

    Order Rodentia

    Order Scandentia

    Order Sirenia

    Order Tubulidentata

    Superorder Xenarthra

    Infraclass Eutheria

    Subclass Theria

    12 ORIGIN AND EVOLUTIONAquatic Temperate

    Forests

    Desert Meadow or

    Pastureland

    Tropical

    Savanna

    Humans have adapted to

    almost all habitats through

    their ability to modify

    certain elements of their

    habitat to their advantage.

    AN UNCOMMON PRIMATE

    Tropical

    Rainforest

    Taiga Tundra

    Take Habitat

    into Account

    Between every mammal and its natural habitat there is a

    relationship that exists and is expressed in the animal's

    physical characteristics. Just as the flippers of the

    elephant seal are used to swim and hunt fish, mimicry and

    running are vital for deer. Physiology is a special

    instrument of adaptation to the environment, as in the

    case of the camel.

    They often create tools to

    help them adapt to their

    environment. In this way,they do not need to rely

    on natural evolution alone.

    Close Relatives

    Humans belong to the primate group. Hominids (orangutans,gorillas, and chimpanzees) are the largest of these, weighing

    between 105 and 595 pounds (48-270 kg). In general, males

    are larger than females, with robust bodies and well-

    developed arms. Their vertical carriage differentiates their

    skeletons from those of other primates. Gorillas inhabit only

    the equatorial jungles of western Africa. They support

    themselves on their forelimbs while walking. Normally

    their height varies between 4 and 6 feet (1.2-1.8 m),but, if they raise their forelimbs and stand erect,they can be over 6.5 feet tall (2 m).

    What Is a Mammal?

    14 ORIGIN AND EVOLUTION

    Mammals share a series of characteristics that distinguish their class: a

    body covered by hair, the birth of live young, and the feeding of

    newborns on milk produced by the females' mammary glands. All

    breathe through lungs, and all possess a closed, double circulatory system

    and the most developed nervous systems in the animal kingdom. The ability

    to maintain a constant body temperature has allowed them to spread out

    and conquer every corner of the Earth, from the coldest climates to hot

    deserts and from the mountains to oceans.

    MAMMALS 15

    MAMMARY GLANDS

    Secrete the milk with

    which the females feed

    their young during their

    first months of life.

    These glands give the

    class its name.

    ALWAYS 98o F

    (37o C)

    The ability to

    maintain a constant

    body temperature is

    not a characteristic

    unique to mammals;

    birds also have that

    ability.

    LOWER JAW

    Formed by a single bone,called the dentary, and

    teeth specialized for each

    function. The entire

    cranium has a very

    simplified bone structure.

    Limbs

    Mammals have four limbs that are adapted for

    moving about on land. Their forelimbs have certain

    other abilities (swimming, manipulation, attack and

    defense, protection). The exceptions are the

    cetaceans, so adapted to marine life that they only

    have two fingerless limbs, and seals (Phocidae).

    ELEPHANT SEALS

    Family Phocidae

    Homeothermy The ability to keep body temperature

    relatively constant, independent of

    the ambient temperature.

    Hibernating species are the

    exception; they must lower

    their body temperature to

    enter into this state of reduced

    metabolic activity. Contrary to

    popular belief, bears do not

    truly hibernate but rather

    enter into a period of deep

    sleep during winter.

    GRIZZLY BEAR

    (BROWN BEAR)

    Ursus arctos

    AN EAR OF BONES

    The tiny bones of the

    ear form a system for

    sensing and

    transmitting sound.

    A Body for Every Environment

    Skin covered with hair and sweat glands helps create and maintain a

    constant body temperature. At the same time, with eyes placed on each side

    of the head (monocular vision, with the sole exception of the primates, which

    have binocular vision), they are afforded important angles of sight. Limbs are

    either of the foot or chiridium type, with slight variations depending on the

    part of the foot used for walking. In aquatic mammals, the limbs have

    evolved into fins; in bats, into wings. Hunters have powerful claws,and unguligrades (such as horses) have strong hooves that support

    the whole body when running.

    A THICK SKIN

    Formed by an outer layer

    (epidermis), another

    deeper layer (dermis),and a fatty substratum

    that contributes to

    homeothermy.

    GORILLA

    Gorilla gorilla

    CRANIUM

    Relatively large

    compared to the size of

    the body. And the brain

    is more developed and

    more complex than that

    of any other animal.

    Hair

    Body hair is unique to mammals and

    absent in other classes of animals.

    Sirenians, with little hair, and cetaceans

    are exceptions; in both cases, the absence

    of hair is a result of the mammal's

    adaptation to an aquatic environment.

    CHIPMUNK

    Family Sciuridae

    BOTTLENOSE

    DOLPHIN

    Tursiops truncatus

    Dentition The majority of mammals change

    dentition in their passage to adulthood.

    Teeth are specialized for each

    function: molars for chewing, canines

    for tearing, and incisors for gnawing.

    In rodents such as chipmunks,the teeth are renewed by

    continuous growth.

    THE NUMBER OF MAMMAL

    SPECIES ESTIMATED TO

    EXIST ON EARTH

    5,416MAMMALS 17

    A Perfect System

    Polar bears, like all mammals, keep their

    internal temperature constant. These

    bears tolerate the extreme cold of the Arctic

    ice because they have developed a

    sophisticated system to increase their ability to

    isolate and capture sunlight. Their transparent

    hair receives a large part of it and therefore

    appears to be white. The hair transmits this

    light inward, where there is a thick layer of

    black skin, an efficient solar collector. Their fur

    is made up of hollow hairs, approximately 6

    inches (15 cm) long, which insulate the bear in

    low temperatures and keep the skin from

    getting wet when in the water.

    AND FINALLY . . .

    THE FLOATING SLAB

    When they tire of swimming,they rest, floating. They manage

    to cross distances of over 37

    miles (60 km) in this manner.

    TO GET OUT:

    ANTISLIP PALMS

    Their palms have surfaces

    with small papillae that

    create friction with ice,keeping them from slipping.

    Hind Legs

    function as

    a rudder.

    Forelimbs

    function as

    a motor.

    RESPIRATORY

    PATHWAYS

    The bears have

    membranes in their

    snouts that warm and

    humidify the air before

    it reaches the lungs.

    SHELTERED CUBS

    The cubs are born in

    winter, and the skin of

    the mother generates

    heat that protects the

    cubs from the

    extreme cold.

    UNDER THE ICE

    Females dig a tunnel in the

    spring; when they become

    pregnant, they hibernate

    without eating and can lose

    45 percent of their weight.

    CHAMBER

    OR REFUGE

    MAIN

    ACCESS

    TUNNEL

    ENTRANCE

    SECONDARY

    ACCESS

    TUNNEL

    PRINCIPAL FAT

    RESERVES

    Thighs, haunches,and abdomen

    HYDRODYNAMIC

    ANATOMY

    LAYERS

    HAIR

    SLOW AND STEADY SWIMMING

    GUARD HAIRS

    Outer

    UNDERFUR

    Inner

    FAT

    4-6 inches (10-15 cm) thick

    An

    impermeable,translucent

    surface

    Hollow chamber

    with air

    Mammals are homeothermic—which means they are capable of maintaining a

    stable internal body temperature despite environmental conditions. This ability

    has allowed them to establish themselves in every region of the

    planet. Homeostasis is achieved by a series of processes that tend

    to keep water levels and concentrations of minerals and

    glucose in the blood in equilibrium as well as

    prevent an accumulation of waste

    products—among other things.

    Constant Heat

    16 ORIGIN AND EVOLUTION

    Great

    Swimmers

    Polar bears swim with ease in open

    waters and reach a speed of 6 miles an

    hour (10 kmh). They propel themselves

    with their great front paws and use their

    back feet as rudders. The bear's hair is hollow

    and filled with air, which helps with buoyancy.

    When the bear dives, its eyes remain open.

    Curling Up

    Many cold-climate mammals curl up into

    balls, covering their extremities and

    bending their tails over their bodies as a

    kind of blanket. In this way, the surface

    area subjected to heat loss will be

    minimal. Hot-climate animals stretch out

    their bodies to dissipate heat.

    Metabolism The layer of fat is between 4 and 6 inches (10-15 cm) thick

    and provides not only thermal insulation but also an energy

    reserve. When the temperature reaches critical levels—at

    the Pole it can drop to between -60° and -75° F (-50° to

    -60° C)—the animal's metabolism increases and begins to

    rapidly burn energy from fat and food. In this way, the

    polar bear maintains its body temperature.

    Migration WHEN SPRING BEGINS, THESE BEARS

    TRAVEL SOUTH, ESCAPING THE BREAKUP

    OF THE ARCTIC ICE.

    6miles (10 km)

    over

    PER HOUR IS THE AVERAGE SPEED AT

    WHICH POLAR BEARS SWIM.

    POLAR BEAR

    Ursus maritimusDEVELOPED SENSES 28-29

    SOFT CONTACT 30-31 What They Are Like

    All mammals have

    stereoscopic vision, which

    gives them depth perception.

    Moreover, in the case of

    hunters such as tigers, their

    night vision is six times keener than

    that of humans. There are many species

    that have a very keen sense of smell,and the sense of taste is closely linked

    to that of smell. Hair, too, performs

    various functions in these animals'

    lives—conserving body heat, providing

    protection, and serving as camouflage.

    Those that have almost no hair and live

    in environments where the

    temperature is very low, such as whales,have developed a layer of fat under their

    skins.

    GRACE AND MOVEMENT 20-21

    EXTREMITIES 22-23

    WHAT DOESN'T RUN, FLIES 24-25

    LOOKS THAT KILL 26-27

    BENGAL TIGER

    Panthera tigris tigris is the largest

    member of the feline family, easily

    recognized by its orange fur with

    black stripes and white spots.50MPH (80KMH)

    Horses, one of the odd-toed, hoofed, ungulate mammals, are considered symbols of grace and

    freedom. They have great vigor and can run swiftly because their spine bends very little,preventing unnecessary expenditure of energy during the rising and falling of their body mass.

    They are equipped with strong, light, and flexible bones, and their muscles work by contraction,arranged in pairs or groups that pull in opposing directions.

    Grace and Movement

    Power to Run

    Horses are one of the most powerful mammals and achieve

    great speeds relative to their body mass. The natural purpose

    of their musculature is to allow them to flee their enemies. This

    ability has allowed the species to survive for millions of years. Their

    great energy is generated by contracting muscles.

    Skeleton

    GALLOPING LEGS

    The hind legs generate the impetus and the leap,and the front legs bear the weight upon landing. To

    save energy, the spine hardly arches when running.

    In felines, however, which are lighter, it does.

    VERTEBRAE

    7 CERVICAL

    TENDONS

    are lengths of connective tissue

    that secure one end of a muscle

    (striated muscle tissue) to a bone

    (bone tissue). Ligaments connect

    bones to one another.

    FROM 17 TO 19

    DORSAL

    Normally there are

    18, but the number

    is often higher or

    lower.

    Ischium

    Ilium

    Tip of

    the Tarsus

    5 OR 6 LUMBAR

    PELVIS

    FIBULA

    TIBIA

    PATELLA

    METATARSUS

    PHALANGES

    RIBS

    RADIUS

    HUMERUS

    ULNA

    KNEE

    METACARPUS

    PASTERN

    FEMUR

    7 SACRAL

    STERNUM

    is the bone that

    joins the ribs in

    the front of the

    chest, forming

    the thoracic cage

    and providing

    visceral support.

    18 COCCYGEAL

    The tail can be made

    up of a variable

    number of very

    mobile vertebrae.

    The medullary canal

    narrows.

    ATLAS

    First cervical vertebra

    is articulated, allowing the nape to

    bend up and down.

    AXIS

    Second cervical vertebra

    allows lateral movement—necessary

    for the horse to turn.

    Atlas

    Correct position of

    an equestrian

    BUCCAL

    CAVITY

    14 TEETH

    in each maxillary

    bone, including:

    3 molars

    3 premolars

    6 incisors

    2 canines

    HOOF

    Because they

    have this kind of

    “nail,” horses are

    called ungulates,as are tapirs and

    rhinoceroses.

    Muscle fascicle

    Muscle fiber

    (cell)

    Perimysium

    Blood Vessel

    Epimysium

    SCAPULAR

    CARTILAGE

    SCAPULA

    EQUINE FOOT

    Metacarpus

    Third Phalanx

    Second Phalanx

    Navicular Bone

    First

    Phalanx

    Sesamoid

    Bone

    Plantar Pad

    Heel

    Bar

    Frog

    Sole

    Horseshoe

    BRACHIALIS

    TRICEPS

    CAUDAL

    DEEP PECTORAL

    MUSCLE

    KNEE

    Lateral Digital

    Extensor

    Twins

    Lateral Band

    Collateral

    Ligament

    THE HORSE IN ACTION

    THE SPEED REACHED

    BY A RUNNING HORSE

    EXTENSOR CARPI

    RADIALIS

    COMMON DIGITAL

    EXTENSOR

    DEEP

    DIGITAL

    FLEXOR

    ORBITAL

    CAVITY

    NASAL

    CAVITY

    ANNULAR

    LIGAMENTS

    DEEP DIGITAL

    FLEXOR TENDON

    PECTORALS

    DELTOIDS

    CLEIDOMASTOIDS

    STERNOCEPHALICUS

    Endomysium

    (between fibers)

    Bone

    20 WHAT THEY ARE LIKE

    34 BONES IN

    THE CRANIUM

    210 IS THE NUMBER

    OF BONES IN

    THE SKELETON

    OF A HORSE

    (excluding the

    tailbones)

    Axis

    MAMMALS 21

    ión bM MAMMALS 23 22 WHAT THEY ARE LIKE

    Extremities

    Mammals' extremities are basically either of the foot or chiridium type but modified

    according to the way in which each species moves about. Thus, for example, they become

    fins for swimming in aquatic mammals and membranous wings in bats. In land mammals,these variations depend on the way the animal bears its weight in walking: those that use the

    whole foot are called plantigrades; those that place their weight on their digits, digitigrades; and

    those that only touch the ground with the tips of their phalanges, ungulates.

    UNGULIGRADE I

    HORSES

    If you observe their

    footprints, you will see

    that only their hooves

    leave marks. Horses'

    hooves are made up of

    only one toe.

    LYING FOOTPRINTS

    Other species of unguligrades (or simply ungulates)

    can have more toes that make up their hooves, but

    they do not place weight on more than two of them.

    HIPPOPOTAMUS PIG CHEVROTAIN DEER CAMEL

    UNGULIGRADE II

    GOATS

    The majority of ungulates,such as goats, have an

    even number of toes. They

    are called artiodactyls as

    opposed to perissodactyls,which have an odd

    number of toes.

    KEY

    TibiaFibula

    Tarsi

    Metatarsi

    Phalanges

    BIG TOE

    NAIL

    DISTAL PHALANX

    MEDIAL

    PHALANX

    PHALANX

    METATARSAL

    CUBOID BONES

    SCAPHOID BONES

    ASTRAGALUS

    CALCANEUS

    TARSI

    SECOND

    TOE

    THIRD

    TOE

    FOURTH

    TOE

    FIFTH

    TOE

    TALUS

    NAIL

    DIGITAL PAD

    PLANTAR PAD

    TOE

    SPUR

    PAD

    METATARSAL

    PAD

    SOLE

    CUNEIFORM BONES

    Medium

    Large

    DIGITIGRADE

    DOG

    These mammals place

    the full surface of

    their toes (or some of

    them) on the ground

    when walking. They

    usually leave the mark

    of their front toes and

    a small part of the

    forefoot as a footprint.

    Dogs and cats are the

    best-known examples.

    RETRACTABLE NAIL

    WALK OR CLIMB

    There is a fundamental difference

    between the human foot and that

    of a monkey. The monkey has a

    long, prehensile digit in its foot

    similar to that in its hand. Monkeys

    use their feet to grab branches as

    they move through the trees.

    ELASTIC LIGAMENT

    When the tendon contracts,this ligament retracts, and

    then the nail does, too.

    EVOLUTION

    It is thought that

    whales descend

    from ancient

    marine ungulates,whose spines

    undulated up and

    down.

    SCAPULA

    HUMERUS

    ULNA

    RADIUS

    CARPI

    METACARPI

    PHALANGES

    FIRST

    FINGER

    SECOND

    FINGER

    THIRD

    FINGER

    FOURTH

    FINGER

    PATAGIUM

    FIFTH

    FINGER

    ULNA

    HUMERUS

    FEMUR

    Calcareous

    Spur

    TIBIA FOOT

    TAIL

    Phalanx

    Medial

    Phalanx

    TENDON NAIL

    Distal

    Phalanx

    Chimpanzee Human

    PLANTIGRADE

    HUMAN

    Primates, and of course

    humans, bear their

    weight on their toes and

    much of the sole of the

    foot when walking,particularly on the

    metatarsus. Rats,weasels, bears, rabbits,skunks, raccoons, mice,and hedgehogs are also

    plantigrades.

    Functionally Adapted

    Another criterion for classifying mammals by their legs, in addition

    to their morphology, is the function the legs perform. Cats, dogs,and horses have four limbs for locomotion. Primates have

    differentiated forelimbs, and they also use legs to capture food or

    bring it to their mouth. Others use legs to swim or fly.

    Felines

    The function of their paws is to

    support their agile and elastic

    bodies, allowing them to move

    about. The front paws also help

    in hunting to catch and hold prey.

    Chiroptera

    From the Greek, meaning

    “winged hand,” this is how bats

    are designated because their

    forelimbs are modified, the

    fingers thinning and

    lengthening to be able to

    support a membrane that

    functions as a wing. The hind

    limbs did not change similarly:

    they have claws.

    Cetaceans

    Whales adapted so well to the sea

    that they seem to be fish. But inside

    their fins —modified front legs—

    there is a bony structure similar to that

    of a hand with fingers. They have no

    hind limbs: the tail, placed horizontally

    and used to move in the water, has no

    connection to those limbs.

    LEFT FOOT OF

    CHIMPANZEE

    Pan troglodytes

    Life-size photo

    5 toes THE NORMAL NUMBER

    FOR MAMMALS:

    RUNNING SPECIES

    HAVE FEWER.

    Tail

    HORIZONTAL IN

    MAMMALS THAT

    SWIM, AS DISTINCT

    FROM FISH

    SmallOrder

    Family

    Species

    Carnivora

    Felidae

    Acinonyx

    jubatus (Africa)

    Acinonyx

    venaticus (Asia)

    NOSTRILS

    Very wide, they allow

    it to receive more

    oxygen as it runs.

    SECOND POINT

    OF CONTACT

    Extending its four

    legs again, it picks up

    more momentum,supporting itself only

    on one back leg.

    FIRST POINT OF CONTACT

    As it runs, only one leg

    touches the ground at a

    time, but during the

    cervical contraction, the

    entire body lifts from the

    ground.

    70 MPH (115 KMH)

    CHEETAH

    It only takes 2 seconds to reach a

    speed of 45 miles per hour (72 kmh).

    18 MPH (29 KMH)

    SIX-LINED RACERUNNER

    Cnemidophorus

    sexlineatus

    23 MPH (37 KMH)

    HUMAN BEING

    Track record: Asafa Powell (Jamaica),110 yards (100 m) in 9.77 seconds

    42 MPH (67 KMH)

    GREYHOUND

    A dog with a light skeleton

    and aerodynamic anatomy

    50 MPH (80 KMH)

    HORSE

    An anatomy designed for

    running, powerful musculature

    BIPEDS VERSUS

    QUADRUPEDS

    ZIGZAGGING

    AT HIGH

    SPEED

    TAIL

    Large compared to

    the rest of the body,it acts as a pivot

    used to suddenly

    change direction.

    TAKEOFF

    From the top of

    a tree, it jumps

    toward another

    shorter tree.

    IN THE AIR

    The flying squirrel does not actually fly—it

    glides. Between its front and back limbs is

    a membrane of skin that, like a delta wing,stretches out the moment the animal

    jumps and stretches its legs. Thanks to

    that it can glide from the top of one tree

    to the trunk of another.

    T Tail

    acts like

    a rudder.

    T TOES

    Upon landing, it grabs

    onto the surface with

    its toes.

    P Patagium

    LANDING

    While gliding, the squirrel

    can change its landing angle.

    Just before landing, it lowers

    its tail and raises its front

    legs, using the membrane

    like an air brake. It lands

    very gently on all four paws.

    SHOULDER

    The extensive

    flexion of the

    shoulder allows

    it to take very

    long leaps.

    LIMBS

    Long and agile. It

    has a powerful,flexible skeleton

    and musculature.

    PAWS

    DIGITS

    5 in the hands

    4 in the feet

    NAILS

    Unlike other

    felines, their

    nails are not

    retractable,allowing them

    to grip the

    ground better.

    HEAD

    Small and

    aerodynamic,with low air

    resistance.

    T

    hey are meteors of flesh, bone, and hot blood. Cheetahs are the fastest of

    the land animals and unique members of the Felidae family, which hunt

    using their keen vision and great speed. They can reach over 70 miles per

    hour (115 kmh) in short runs and reach 45 miles per hour (72 kmh) in an

    average of only 2 seconds. They can get above 60 miles per hour (100 kmh),but they can sustain that speed for only a few seconds. They look like leopards,although their physical characteristics are different: they are longer and

    thinner, and their heads are smaller and rounded.

    What Doesn't Run, Flies

    24 WHAT THEY ARE LIKE

    Cheetahs

    Whereas tigers prefer to lie in wait for

    prey and then jump on it, the cheetah

    uses explosive speed of over 60 miles per

    hour (100 kmh) to run its prey down.

    Siberian

    Flying

    Squirrel

    Flying squirrels (Pteromys volans)

    belong to the same rodent family

    as common squirrels, to which

    they are similar in both

    appearance and way of life. They

    live in the mixed forests of

    northern Europe, across Siberia,and into East Asia.

    1 START

    The cheetah

    begins running

    by lengthening

    its stride and

    extending its

    four legs.

    2 SPINAL

    CONTRACTION

    Then it gathers its legs

    under its body, contracting

    its cervical spine to the

    maximum.

    3 EXTENDING

    THE SPINE

    In a counterthrust opposing

    the contraction, the spine

    extends, creating forward

    momentum. The cheetah

    can cover 26 feet (8 m) in a

    single stride.

    70 (115 kmh)

    MAXIMUM SPEED, BUT CAN

    BE MAINTAINED FOR ONLY

    550 YARDS (500 M)

    miles

    per hour

    Sloth

    These animals are notable for their

    extremely slow metabolism. They

    take half a minute to move a

    limb! They are also somewhat

    myopic, their hearing is

    mediocre, and their sense

    of smell barely serves to

    distinguish the plants

    on which they feed.

    They are at the extreme

    opposite of cheetahs.

    However, since they

    practically live perched in

    trees, they do not need to

    move or see or hear precisely.

    They are perfectly adapted to

    their way of life.

    THREE-TOED SLOTH

    Native to the Amazon River basin

    These movements

    are possible

    because its nails are

    not retractable, so

    that cheetahs

    firmly grip the

    ground.

    2

    Cheetahs can

    make sharp turns

    while running at

    high speed.

    1

    MAMMALS 25Field of Vision

    FIELD OF VISION 50 times

    Seeing Even in the Dark

    Hunting animals depend on the keenness of their senses to detect their prey.

    Felines can dilate their pupils up to three times more than humans, and they

    see best when light is dim and their prey's movements are very subtle. A system

    of 15 layers of cells forms a sort of mirror (tapetum lucidum) located behind the

    retina or back of the eye. This mirror amplifies the light that enters and is also the

    reason that the animal's eyes shine in the dark. At the same time, their eyes are

    six times more sensitive to light than those of people. Tigers' nocturnal vision also

    increases because of the great adaptability of their circular pupils when they are

    completely open.

    BINOCULAR

    VISION

    Part of the field

    of vision of one

    eye overlaps that

    of the other eye,which makes

    three-dimensional

    vision possible.

    Hunters' skills

    depend on

    binocular vision,because it allows

    them to judge the

    distance and size

    of their prey.

    PUPILS

    They regulate the passage of light

    to the retina by contracting in

    bright light and dilating in the dark.

    In each species of mammal, the

    pupils have a distinctive shape.

    HUMAN DOG WITH LONG

    SNOUT

    HARE SHORT-SNOUTED DOG

    LIGHTS OR COLORS

    The retina's

    sensitivity to light

    depends on rod-

    shaped cells, and

    forms and colors

    depend on other

    cells, which are

    cone-shaped. In

    tigers, the former

    predominate.

    RETINA OF A

    NOCTURNAL

    ANIMAL

    Rods, super-

    sensitive to light,predominate.

    RETINA OF A

    DIURNAL ANIMAL

    Cones, which

    distinguish colors

    and details, along

    with light,predominate.

    ROD CONE

    Tigers have a 255°

    angle of vision, of

    which 120° is

    binocular, whereas

    humans have 210° with

    120° of it binocular.

    FOCUS 2

    FOCUS 1

    THE LIGHT AMPLIFICATION

    CAPABILITY OF THE RETINA

    OF FELINES

    Left

    Field

    Right

    Field of

    Vision

    TIGER CAT GOAT

    Binocular

    Field

    T

    igers are the largest of the world's felines. Predators par

    excellence, they have physical skills and highly developed senses

    that they use to hunt for prey. Their daytime vision is as good as

    that of humans, except for a difficulty in seeing details. However, at

    night, when tigers usually hunt, their vision is six times keener than that

    of a human being, because tigers' eyes have larger anterior chambers

    and lenses and wider pupils.

    Looks That Kill

    26 WHAT THEY ARE LIKE

    CONJUNCTIVA

    CORNEA

    LENS

    IRIS

    PUPIL

    VITREOUS

    HUMOR

    OPTIC

    NERVE

    RETINAHearing

    The auditory ability of dogs is four times greater than that of

    human beings, and it is highly developed. Their ability depends

    on the shape and orientation of their ears, which allow them to

    locate and pay closer attention to sounds, although this varies

    by breed. They can hear sharper tones and much softer sounds,and they can directly locate the spatial reference point where

    a noise was produced. Dogs hear sounds of up to 40 kilohertz,whereas the upper limit for human hearing is 18 kilohertz.

    Sense of Smell

    Their most developed sense; they have

    220 million olfactory cells in their nasal

    cavities. Mucous tissue, located in the

    nasal conchae of the snout, warms and

    moistens the air that they inhale.

    Taste

    Dogs perceive the chemical substances that

    foods are made of by means of receptor cells

    found in the taste buds located at the back of

    the tongue and in the soft part of the palate.

    LABYRINTH

    SEMICIRCULAR

    CANALS

    AUDITORY

    OSSICLES

    INCUS (ANVIL)

    MALLEUS (HAMMER)

    STAPES (STIRRUP)

    INSIDE THE COCHLEA

    INTERNAL

    STRUCTURE OF

    THE BULLA

    The dome diverts

    sounds toward the

    bulla, which sends

    electric signals to

    the brain.

    TURBINATE BONES

    The epithelium that

    covers these bones is

    responsible for

    secreting mucus that

    traps inhaled particles.

    THE TONGUE

    AND TASTES

    Sweet tastes are

    experienced in the

    front part of the

    tongue, sour ones

    in the center, and

    salty ones in the

    back. On either

    side salty and

    sweet are mixed.

    TASTE

    RECEPTORS

    Individual receptor

    cells pass

    information to the

    olfactory centers of

    the brain.

    AUDITORY LEVELS

    TASTE BUDS

    Dispersed throughout

    the tongue. Complex

    interactions among

    them determine taste by

    means of nerve endings.

    Fragrant

    Material

    Dendrites

    Mucous

    Layer

    Receptor

    Cell

    Nerve

    Fiber

    Dogs have inherited from wolves great hearing and an excellent sense of smell. Both perform

    an essential role in their relationship to their surroundings and many of their social

    activities. However, they are very dependent on the keenness of their senses depending on

    the habitat in which they develop. Whereas humans often remember other people as images,dogs do so with their sense of smell, their most important sense. They have 44 times more

    olfactory cells than people do, and they can perceive smells in an area covering some 24

    square inches (150 sq cm). Dogs can discern one molecule out of a million other ones,and they can hear sounds so low that they are imperceptible to people.

    Developed Senses

    28 WHAT THEY ARE LIKE

    Reissner's

    Membrane

    Organ

    of Corti

    Scala

    Tympani

    Dome

    Crest

    Ciliary

    Cells

    People

    Foxes

    Mice

    Bats

    Frogs

    Elephants

    Birds

    0 hertz 1 10 100 1,000 10,000 20,000 40,000

    Scala

    Vestibuli

    AUDITORY

    NERVE

    AUDITORY CANAL

    TYMPANIC MEMBRANE

    COCHLEAR

    NERVE

    AURICULAR

    CARTILAGE

    AUDITORY

    CANAL

    COCHLEA

    MIDDLE EAR

    SALTY

    SOUR

    SWEET

    OVAL

    WINDOW

    EUSTACHIAN

    TUBE

    COCHLEA

    1,000

    over

    times

    THE CAPABILITY OF A DOG'S SENSE

    OF SMELL COMPARED TO THAT OF

    A HUMAN

    SALTYSWEET

    SALTYSWEET

    M MAMMALS 29FUR SERVES TO PROTECT

    THE SKIN FROM

    EXCESSIVE UV RAYS. UV

    30,000 THE NUMBER OF QUILLS THAT

    COVER A PORCUPINE (148 PER

    SQUARE INCH [23 PER SQ CM])

    1

    Soft Contact

    EPIDERMIS

    Outer layer

    formed by

    resistant, flat

    cells

    HAIR

    STRUCTURE

    DERMIS

    Layer with blood

    vessels, glands,and nerve endings.

    It is a layer of

    sebaceous glands

    that secrete an

    oily substance,sebum, on the

    surface of the skin.

    STRATUM

    CORNEUM

    FATTY TISSUE

    This is a specialized

    conjunctive tissue

    made up primarily of

    connective cells called

    adipocytes, which

    store energy in the

    form of triglycerides.

    SWEAT GLANDS

    When the body is hot, the glands

    secrete sweat, which passes

    through the sweat ducts to the

    surface of the skin.

    Fur and Mimicry

    Mammals from cold regions, such as

    polar bears, have white fur to camouflage

    themselves in snow. Others, such as polar,or Arctic, foxes and the American hare,change their fur color with the seasons,because they live in areas that are snow-

    covered in winter, where their brown

    summer fur would make them easy prey.

    Lions' beige color helps them avoid being

    discovered while they stalk their prey.

    Diverse Hairs

    The majority of mammals' fur is

    made up of more than one type of

    hair, and its different colors are due

    to a group of proteins called

    melanins. Each coat has different

    layers. Guard hairs are the first layer,providing protection. Underneath

    that, there is a fine layer called

    underfur, formed by constantly

    growing short hairs that renew

    the coat.

    Insulating

    Skin

    Insulation is one of the functions

    of animals' skins and hair. It not

    only helps to conserve body

    warmth but also, as in the case

    of camels, protects them from

    excessive heat. Its color often

    blends in with its surroundings,serving as camouflage.

    The Skin

    SUMMER

    The fur coat of the

    Arctic fox (Alopex

    lagopus) in summer is

    half as thick as that of

    winter, with less than

    half the underfur. In

    summer, “white” phase

    animals turn a gray-

    brown to grayish color,and those that have a

    “blue” phase are

    browner and darker.

    WINTER

    Arctic foxes have two

    kinds of color phases.

    White phase foxes are

    almost pure white in

    the winter, which

    allows them to

    camouflage themselves

    in the snow and ice.

    GREY WOLF HARE CHINCHILLA MACAQUE

    MONKEY

    COATI SEA

    LION

    (JUVENILE)

    PORCUPINE

    30 WHAT THEY ARE LIKE

    SWEAT

    PORE

    Microfibrils

    Macrofibrils

    Cortex

    Medulla

    Scaly Cuticle

    RUFFINI'S

    CORPUSCLE

    ARRECTOR PILI

    MUSCLE

    FOLLICLE

    VEIN

    ARTERY

    HAIR

    SHAFT

    DERMAL PAPILLA

    attaches the

    dermis to the

    epidermis.

    BAT HAIR

    Each strand of hair

    has an outer

    cuticle formed by

    superposed scales.

    ENLARGED

    WOOL

    This is the

    most complex

    natural textile

    fiber in existence.

    It absorbs moisture

    but repels water.

    Mini-quills

    Sharp scales

    PORCUPINE QUILLS

    Called guard hairs, they are

    located outside the fur. In the case

    of the porcupine, they have been

    modified to form defensive quills.

    WOOL FIBER

    Protofibril

    Microfibril

    Macrofibril

    Cortex 90%

    Cuticle 10%

    POLAR

    BEAR HAIR

    Each one of its

    hairs is hollow and

    filled with air. This

    heightens the

    insulating capability

    of the inner layer.

    MERKEL'S DISK

    A sense receptor

    under the skin's

    surface that

    responds to light,continuous touch

    and pressure

    SEBACEOUS

    GLAND

    secretes a waxy

    substance, or

    sebum, which

    moistens the skin,making it

    waterproof.

    PACINIAN

    CORPUSCLE

    Sense receptors

    under the dermis.

    The Pacini receptors

    lie under the layer

    of deep fat and

    detect vibration and

    pressure.

    ERECTION

    MECHANISM

    When the quill

    touches a strange

    surface, it exerts a

    light downward

    pressure on the

    epidermis.

    The fine tissue

    that covers the

    root of the quill

    breaks.

    The erector pili

    muscle receives

    the contact signal

    and contracts.

    Root

    Connective

    Tissue

    Retinaculum

    Epidermis

    Base of

    the Quill OUTER

    FUR

    UNDERFUR

    LAYER OF FAT

    2

    3

    MAMMALS 31

    Admired, adored, and coveted by humans, a mammal's fur coat is much more than

    a skin covering. It acts as a protective layer against mechanical injuries, prevents

    invasion by germs, and regulates the loss of body heat and moisture. In many

    species, such as the Arctic fox, it provides camouflage by changing color and texture

    from winter to summer.HERBIVORES 52-53

    THE GREAT CHAIN 54-55

    ONE FOR ALL 56-57

    WOLVES IN SOCIETY 58-59

    THE FIRST DAYS 44-45

    TRADEMARK 46-47

    DEVELOPMENT AND GROWTH 48-49

    OF FLESH THOU ART 50-51

    Behavior and Life Cycle

    Mammalian reproduction is

    sexual and by internal

    fertilization, which

    involves copulation

    between the male and the

    female. Mammals are also characterized

    by the offspring's dependence on its

    parents. In any case, there is a group of

    mammals called monotremes that is

    oviparous; that is, its members

    reproduce by laying eggs. Mammalian

    behavior consists of a mixture of

    inherited components and components

    that can be shaped by learning. Part of

    this process is accomplished through

    play, since the young use such

    encounters to practice jumping, biting,hunting, and other survival skills. You

    will discover this and much more when

    you turn the page.

    LIFE CYCLE 34-35

    BEAUTY AND HEIGHT 36-37

    OVIPAROUS MAMMALS 38-39

    EFFICIENT NURSERY 40-41

    MIRACULOUS PLACENTA 42-43

    EAT TO LIVE

    An hour after birth, the

    giraffe gets up and with its 8

    feet (2.5 m) of height begins

    to take its first steps in

    search of its mother's teat.17

    Marsupials

    Very short gestation period, after

    which they develop in a sort of partially

    open pouch (the marsupium), which the

    female carries on her belly. The majority

    of the roughly 300 known species of

    marsupials are solitary, except in mating

    periods. In general, they are promiscuous

    animals, although some, such as wallabies

    (small kangaroos), tend to mate with the

    same female all their life.

    Monotremes

    Mammals whose females lay eggs are generally

    solitary species for most of the year. Platypuses

    are seen as couples only when they mate.

    Although they have a period of courtship for

    one to three months, the males have no

    relationship with the females after

    copulation or with the offspring. Short-

    beaked echidna females practice

    polyandry, copulating with various

    males in various seasons.

    Birth, maturity, reproduction, and death: this life cycle has

    certain particularities among mammals. As a general rule,the larger a mammal, the longer the members of its

    species tend to live but the fewer offspring are born to a single

    female per litter or reproductive season. Most mammals,including humans, are placental mammals; their vital functions

    are fully developed inside the body of the mother.

    EASTERN

    COTTONTAIL

    RABBIT

    Sylvilagus

    floridanus

    SHORT-BEAKED

    ECHIDNA

    Tachyglossus

    aculeatus

    KOALA

    Phascolarctos

    cinereus

    AT BIRTH

    The young weigh

    some 1.5 to 1.8

    ounces (40-50 g).

    They do not open

    their eyes until the

    10th day.

    BANISHED

    OFFSPRING

    Dominant males keep

    the offspring and other

    young males apart.

    NUMBER OF OFFSPRING

    In general, it is inversely

    proportional to the species' size.

    LONGEVITY

    GESTATION PERIODS

    COMPARISON

    OF EGG SIZE

    Lactation

    25 TO 30 DAYS

    fed upon milk, although

    they can digest solid food

    after 20 days. The young

    abandon the burrow after

    35 or 40 days and remain

    in the area where they

    were raised (philopatry).

    Weaning

    35 TO 40 DAYS

    Young rabbits remain with their

    mother even after nursing ends

    for protection and the inculcation

    of species-specific behavior.

    Sexual

    Maturity

    5 TO 7 MONTHS

    The better rabbits are fed,the more quickly they become

    capable of reproducing. They

    are considered adults at 8 or

    9 months, when they weigh

    some 2 pounds (900 g).

    Lactation

    22 WEEKS

    A muscle inside the pouch

    prevents the infant from

    falling out. At 22 weeks,it opens its eyes, and a

    type of pap produced by

    its mother is added to its

    diet, which will prepare it

    for an herbivorous diet.

    Gestation

    35 DAYS

    With its extremities and

    functional organs barely

    developed at birth, the

    newborn must crawl by itself

    from the cloaca to the pouch

    to continue its development.

    Incubation

    12 DAYS

    Eggs gestate for a month

    before hatching. They

    incubate within a pouch

    for about 10 days to

    remain at the proper

    temperature until the

    young are born.

    Leaving the Pouch

    1 YEAR

    The offspring reaches a size that allows it

    to fend for itself. It has already

    incorporated herbivorous food into its

    diet. The mother can become pregnant

    again, but its young will remain nearby.

    Sexual

    Maturity

    3 TO 4 YEARS

    At two years, koalas

    already have developed

    sexual organs (females

    earlier than males). But

    they do not start

    mating until one or two

    years later.

    Longevity

    4 to 10 years

    Longevity

    15 to 20 years

    Gestation

    28 TO 33 DAYS

    They spend it in a collective

    burrow (warren) dug in the

    ground and covered with

    vegetation and fur. The

    female will abandon it as

    soon as lactation ends.

    Cow

    Goat

    Dog

    Rat

    1 OFFSPRING

    2-3

    OFFSPRING

    5-7

    OFFSPRING

    6-12

    OFFSPRING

    3 to 9

    Young

    PER LITTER, AND

    FROM 5 TO 7

    LITTERS PER YEAR

    1 to 3

    EGGS AT A TIME

    People

    Elephants

    Horses

    Giraffes

    Cats

    Dogs

    Hamsters

    Giraffes

    Gibbons

    Lions

    Dogs

    Elephants

    23

    9

    7

    2

    ANIMAL MONTHS

    70 years

    70

    40

    20

    15

    15

    3

    Life Cycle

    34 BEHAVIOR AND LIFE CYCLE

    Placental Mammals

    This is the largest group of mammals, the one that has

    multiplied most on the planet, although its form of

    gestation and lactation produces great wear and tear

    on the females, making them less prolific. They are

    generally polygenetic: a few males (the most

    competitive) fertilize many females, and other

    males, none. Only 3 percent of mammals are

    monogamous in each season. In these

    cases, males participate in rearing the

    offspring, as they also do when resources

    are scarce. If resources are abundant,the females take care of the young alone,and the males mate with other females.

    90 Years A WHALE'S AVERAGE LIFE SPAN—THE

    GREATEST OF ANY LIVING MAMMAL

    They are born

    without fur, with

    semitranslucent

    skin.

    They have

    four to five

    pairs of

    breasts.

    They make

    use of natural

    caves or dig

    underground.

    Female

    rabbits can

    mate at

    any time.

    By the end of

    lactation, fur

    covers the

    whole body.

    Newborn

    Offspring

    Underground cave or

    a cave among rocks

    Undeveloped

    Limbs

    The fur is

    already spiny.

    Shell

    Dominant

    males mate

    with all the

    females.

    Some females

    leave to look for

    strong males.

    The young animal fastens itself to

    its mother and is carried around

    by her, clinging to her shoulders.

    1 offspring

    1 BIRTH PER YEAR

    0.8 inch

    (2 cm)

    In the Pouch

    2 TO 3 MONTHS

    After breaking the shell, the

    young are suckled while they

    remain in a kind of pouch of

    the female.

    0.5 inch

    (15 mm)

    Weaning

    4 TO 6 MONTHS

    After three months,the offspring can leave

    the burrow or remain

    in it alone for up to a

    day and a half before

    finally separating from

    the mother.

    Longevity

    50 years

    MAMMALS 35

    The shell is soft

    and facilitates the

    offspring's birth.

    Unlike birds, they

    do not have beaks.

    Chicken

    Echidna

    4 inches

    (10 cm). Red Deer

    These are svelte, robust, well-

    formed animals with a majestic

    and haughty carriage. They are very

    timid and fearful, and it is thought that

    the species is 400,000 years old. They

    are active at daybreak and evening,and males usually live alone. Females

    and younger deer group in herds.

    Order

    Family

    Species

    Diet

    Weight

    (male)

    Artiodactyla

    Cervidae

    Cervus elaphus

    Herbivorous

    400 pounds

    (180 kg)

    F

    inding a female with whom to mate is the great

    effort of the male's life, a competition with other

    males of his own species. Each animal has its

    particular nuances. For stags, antlers play a fundamental

    role in winning the heart of their chosen one. Whichever

    stag has the most beautiful, longest, and sharpest horns

    will be the winner. Thus, he will be able to defend his

    territory, court the female, and reproduce.

    Beauty and Height

    36 BEHAVIOR AND LIFE CYCLE

    Fights

    When two males fight over a harem,each will display his antlers to

    frighten his rival. The horns can also

    be used to defend against predators.

    Molt ......